I don't think that it will ever be possible to automate all web accessibility checking. If you know the checker well enough, you will probably be able to create a test-page which passes the test but is inherently inaccessible. Even if you don't know the checking system, you will probably be able to construct such a page, as Gez Lemon of JuicyStudios found when he tried Testing Invalid Content with Accessibility Validators.

The bottom line will always be that these tools have a real place as a first "quick and dirty" check, but real user-testing and/or (shameless plug) an Accessibility Audit by an experienced Web Accessibility Expert cannot be replaced.